docs: def getCancelURL(self):
          """Get the URL to cancel this request.
  
          Useful for creating a "Cancel" button on a web form so that operation
          can be carried out directly without another trip through the server.
  
          (Except you probably want to make another trip through the server so
          that it knows that the user did make a decision.  Or you could simulate
          this method by doing C{.answer(False).encodeToURL()})
  
          @returntype: str
          @returns: The return_to URL with openid.mode = cancel.
  
          @raises NoReturnError: when I do not have a return_to.
          """
          if not self.return_to:
              raise NoReturnToError
  
          if self.immediate:
              raise ValueError("Cancel is not an appropriate response to "
                               "immediate mode requests.")
  
          response = Message(self.message.getOpenIDNamespace())
          response.setArg(OPENID_NS, 'mode', 'cancel')
          return response.toURL(self.return_to)